The invention relates to an internal combustion engine with a multi-joint crank drive, which includes a plurality of coupling members which are rotatably supported on crank pins of a crank shaft and a plurality of articulation connecting rods which are rotatably supported on crank pins of an eccentric shaft, wherein each of the coupling members is pivotally connected to a pivotal piston connecting rod of a piston of the internal combustion engine and to one of the articulation connecting rods, characterized by a single balance shaft which serves for neutralizing second order inertia forces.
Such Internal combustion engines are for example known from DE-A-102005054761, DE-A-102005054760, EP-A-1126144, JP-A-2004124775 or WO-A-2007057149 and are often referred to as internal combustion engines with variable compression ratio or as internal combustion engines with variable piston stroke (stroke variable engine).
These internal combustion engines include an external shaft which is coupled to the crank shaft via a multi-joint crank drive. The multi-joint crank drive includes coupling members whose number corresponds to the number of cylinders, which coupling members are each rotatably supported on a crank pin of the crank shaft and two arms which protrude over the crank shaft and are provided at their ends with a pivot joint. One of the pivot joints serves for pivotally connecting a piston connecting rod which connects one of the pistons of the internal combustion engine to the crank shaft via the coupling member, while the other pivot joint serves for pivotally connecting a so called articulation connecting rod which is rotatably supported with its end on a crank pin of the eccentric shaft.
Similar to conventional internal combustion engines without an eccentric shaft which is coupled to the crank shaft via a multi-joint crank drive, second order inertia forces are also generated in internal combustion engines of the previously mentioned type as a result of oscillating masses and can change with the crank angle. For achieving a desired very smooth running and for decreasing noise these inertia forces have to be compensated as far as possible. While the first order inertia forces can be compensated by compensating weights on the crank shaft and by the crank sequence, the compensation of second order inertia forces in conventional internal combustion engines often occurs by means of two compensation shaft which rotate in opposite direction and which are driven with twice the rotational speed as the crank shaft.
However, internal combustion engines of the previously mentioned type already have higher frictional losses than conventional internal combustion engines, so that the additional frictional losses of two compensation shafts would exceed an acceptable limit.
In order to solve this problem the above mentioned JP-A-2004124775 already proposes to improve the mass compensation by favorable positions of the centers of gravity of the components of the multi-joint crank drive, which however, is not advantageous with regard to the second order inertia forces because the crank drive masses become even higher by the optimized centers of gravity.
Further, the previously mentioned EP-A-1126144 describes how the second order inertia forces in a 4 cylinder internal combustion engine in serial construction with a multi-joint crank drive can be brought below a defined threshold value by optimizing the kinematic, wherein however no reference to an improved smooth running can be found.
Other measures which are described in some of the previously mentioned documents such as in DE-A-102005054761, DE-A-102005054760 or WO-A-2007057149 also only allow for very limited compensation of the second order inertia forces, so that series internal combustion engines with multi-joint crank drive with regard to smooth running a inferior to conventional series internal combustion engines.